Pressure sensors have become ubiquitous the last several years, finding their way into many applications, such as tire pressure monitor systems (TPMS), manifold absolute pressure (MAP) applications, for example, in automotive exhaust lines, automatic transmission gear boxes, consumer applications involving barometric pressure measurements, and others.
These pressure sensors may typically be formed on a silicon die having a diaphragm over a cavity, where the diaphragm is supported by a sidewall or bulk region. One or more sensors, such as a Wheatstone bridge consisting of diffused or implanted resistors, may be located on the diaphragm. Pressure, either from above the diaphragm or below in a cavity, deflects the diaphragm and its sensors. This deflection, and hence the pressure, can be measured by utilizing the piezo-resistive effect. That is, as the sensors are stressed due to the bending of the diaphragm, the piezo-resistive effect changes the value of one or more of the resistors that are typically configured in the Wheatstone bridge. This change in resistance results in a change in the output of the Wheatstone bridge from which the change in pressure can be inferred.
Pressure sensor performance can be improved by increasing the sensor's sensitivity. Increased sensitivity results in a larger output signal that is less affected by or compromised by noise and signal interference. Sensor performance can also be improved by improving its linearity. Improved linearity results in a more accurate pressure to output signal conversion. Unfortunately, increasing either sensitivity or linearity comes at the expense of the other.
One way to improve both sensitivity and linearity is for a portion of the diaphragm to be formed having a relatively thicker portion, which may be referred to as a boss. This boss may extend into the cavity below the diaphragm. The boss may act as a concentrator, concentrating pressure induced stress into a smaller area, thereby increasing device sensitivity. This concentration may also increase linearity. But many problems may arise with the use of this type of boss.
For example, since the boss is formed in the cavity below the diaphragm, it may be difficult to align the boss with features such as a Wheatstone bridge on the topside of the diaphragm. Additionally, manufacturing processes for said boss have to be performed on the backside of the silicon wafer, which is disadvantageous compared to standard CMOS manufacturing techniques. Also, since these bosses are typically thick, they have a relatively large mass. This mass may provide stress on the diaphragm when it is accelerated, such as when the pressure sensor is moved. Gravity may cause the boss to provide a varying stress on the diaphragm as the pressure sensor is placed in different orientations. Further, bosses are typically formed using a potassium hydroxide (KOH) etch. These etches result in a cavity having sloped sides. Sloped sides mean a greater area is consumed by the cavity, which increases the size of the pressure sensor. Moreover, an n-doped silicon wafer is needed for a KOH etch. Unfortunately, most CMOS processes are based on using a p-doped silicon wafer.
Thus, it is desirable to provide pressure sensors having bosses that have a reduced mass and are more readily aligned to features on a top side of a diaphragm. It is also desirable to have a pressure sensor with a cavity formed to provide a smaller die size in a manner compatible with conventional CMOS processes.